Saturday, December 28, 2013

Since more than 2000 years ago explorers have marveled at “red snow,” a snow notable for its characteristic tinge that is typically encountered in the alpines or the polar regions of the world (Remias, Lütz-Meindl, & Lütz, 2005). With recent advances in
molecular biology scientists have determined that of all the microorganisms
that exist within these habitats, snow algae are responsible for this
phenomenon, as well as similar green, yellow, orange, and grey tinges in the
snow (Remias et al., 2005). These colors have been attributed to a variety
of carotenoids within the snow algae, which are pigmented molecules that are
commonly utilized for obtaining light energy at a range of wavelengths during
photosynthesis (Kvíderová, 2012; Remias et al., 2005). Although, these carotenoids
are believed to play an additional role in the snow algae, as their presence
has been implicated in enabling snow algae to resist cold (Remias, Karsten, Lütz, & Leya, 2010).

The snow algae are psychrophiles,
capable of thriving at the freezing temperatures of the frigid tundras of the
world. Of these algae, the genus Chlamydomonas
makes up most of the pigmented microbes in “red snow” (Remias et al., 2005). Since the physiologies that snow algae maintain
to tolerate this environment likely differ between different microorganisms,
the studied eukaryote Chlamydomonas
nivalis will be the focus of this blog post. C. nivalis often contains large amounts of carotenoid-associated
vesicles in its cytoplasm. Of these pigments in C. nivalis, the most common carotenoid is astaxanthin, which has
been attributed to the presentation of “red snow” (Hoham & Duval, 2001). The interesting connection
between the caroteniod-associated vesicles and C. nivalis’ psychotropic physiology is that C. nivalis tends to dramatically increase production of these
vesicles and carotenoids when it forms hypnoblasts, an immobile form that is
resistant to cold temperatures (Hoham & Duval, 2001; Remias et al., 2005). These hypnoblasts contain
large quantities of unsaturated lipids on both their intra and plasma membranes,
which makes them better able to avoid solidifying at low temperatures (Remias et al., 2005). The function of the associated carotenoids is
less clear, and this will be a major discussion point of this post.

Researchers
were curious about astaxanthin in C.
nivalis, so they set out to determine how C. nivalis hypnoblast cells’ rate of photosynthesis is affected by
temperature and hypothesized how astaxanthin fits in to the results (Remias et al., 2005). These researchers subjected C. nivalis in its hypnoblast form to a
variety of temperatures and measured its rate of photosynthesis. As a result,
they found that C. nivalis actually
had a significantly higher photosynthetic rate at moderate temperatures than
the low temperatures it would typically be found within in its hypnoblast form (Remias et al., 2005). I thought this was curious: why would an
microbe that is essentially just trying to survive stressful conditions be so
well suited to less stressful moderate temperatures? Clostridium, for example, forms highly resistant, inactive spores
in harsh conditions that germinate under more permissive conditions (Wiegel, Tanner, & Rainey, 2006). Clearly this is not the case
with C. nivalis, but why would such
adaptability and activity be favorable for this stressed microbe? The answer to
this question lies in the specific environment that C. nivalis exists in. In the arctic, the sun tends to melt the top
layer of snow during the day. This top layer of melted snow refreezes during
the night (Remias et al., 2005). This constant cycle of melting and freezing
creates a significantly different microbial habitat than one that is constantly
cold. While in a consistently cold environment a microorganism would likely be
better off maintaining low activity (or even no activity as it waits for better
conditions to arise) to survive, C.
nivalis is able to take advantage of exposure to the sun (using the energy
for photosynthesis) during the thaw cycles (Remias et al., 2005). The researchers connect the photosynthetic
capabilities of hypnoblasts to carotenoids by stating that the carotenoids
protect the photosynthetic apparatus by absorbing harmful light rays like UV
rays (Remias et al., 2005). This made some sense to me, as carotenoids are
known to fulfill this protective function as well as absorbing different
wavelengths for photosynthesis. But, why ramp up production of these
carotenoids after forming a hypnoblast and not before? Don’t C. nivalis vegetative cells need
protection from harmful sunrays as well?

Although I
do not have definitive answers to these questions, information covered in this
next paper provides some food for thought regarding this topic. 5 years after
the previously discussed paper, researchers from that paper again looked at C. nivalis hypnoblast cells. However,
this time they were concerned mainly with how the hypnoblasts as a whole
present under stressful conditions (Remias et al., 2010). The researchers noted that upon forming the
hypnoblasts the cells grew about 30 % in size, lost their flagella, and stopped
dividing completely. The researchers also found that in addition to the
increased number of carotenoid-associated vesicles in hypnoblast compared to
vegetative cells, the chloroplasts also decreased in size significantly. Other
cytoplasmic changes indicated a decreased metabolism, including alterations to
the nucleus, ER, and Golgi (Remias et al., 2010). Generally speaking, these changes are indicative
of reduced activity, which is consistent with how most microbes function in a
stressful environment. Again, the researchers assert that the main function of
secondary carotenoids like astaxanthin is protection from harmful radiation
such as UV radiation (Remias et al., 2010). However, the question still remains: why the
massive increase of secondary carotenoids in hypnoblasts from vegetative cells?
As stated above, there are significant changes to the host cytoplasm following
the hypnoblast transformation, one of which being the formation of numerous
large vesicles. These vesicles take up most of the cytoplasm, and one seemingly
significant benefit to using astaxanthin as a protective pigment is that it
forms mono-esters with the unsaturated fatty acids that make up the vesicles,
which causes astaxanthin to associate tightly with the vesicles (Remias et al., 2010). Therefore, it would make sense that astaxanthin
(and potentially other similar carotenoids) is an effective protective pigment
in hypnoblasts because it packs efficiently in an environment where space is
limiting. Other researchers have hypothesized that since certain proteins that
protect the cell from harmful radiation are produced at high concentrations
only when a cell is photosynthetically active, secondary astaxanthin fills this
void for the cell when it is not photsynthetically active in its hypnoblast
form (Hoham & Duval, 2001).

In situations
like these where there are only a handful of primary literature papers on the
topic of interest there are plenty of questions and few answers regarding the
topic. There seems to be a lot different processes at work when C. nivalis forms its cold resistant
hypnoblast form, and the literature has only scratched the surface. One of the
fun things about these circumstances is that there is plenty of room for
hypothesizing, and, with adequate resources, testing these hypotheses. There
seems to be a tight link between the “red snow” first observed by explorers
more than 2000 years ago and the ability of C.
nivalis to persist as a psychrophile, but further research is required to
truly uncover this mysterious link.

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